This invention concerns silicone oils useful as defoaming agents and more particularly to polyoxyalkylene-polysiloxane block copolymers useful for defoaming aqueous dispersions of organic substances and to a method of defoaming therewith.
The use of silicone oils, especially methylpolysiloxanes of low to moderate viscosity, for defoaming aqueous solutions or dispersions is well known and described, for example in Chemie und Technologie der Silicone (Chemistry and Technology of the Silicones) by W. Noll, 1968, page 540 ff.
The improvement in the defoaming action of organocarbon or organosilicon defoamers by the addition of highly disperse inorganic or organic substances, especially of highly disperse usually pyrogenically produced silica and highly disperse aluminum oxide is also known from the literature and mentioned, for example, in German Patent 1,067,003 and German Offenlegungsschrift 1,914,684.
The use of polyoxyalkylene-polysiloxane block copolymers as defoaming agents is also well known. For example, it is stated in the German Patent 1,012,602 that water soluble block copolymers of the general formula, EQU R'[R.sub.2 SiO).sub.y ].sub.a [(C.sub.n H.sub.2n O).sub.x ].sub.b R"
wherein R' and R" represent monovalent hydrocarbons or hydrocarbon groups, y is a whole number of at least 2, n is a whole number from 2 to 4, x is at least 5, and the sum of a and b is equal to 2 or 3, can be used as antifoaming agents. The polyoxyalkylene-polysiloxane block copolymers described in the German Patent 1,012,602 may have the structure A'B'A', wherein A' represents the polyoxyalkylene blocks and B' a polysiloxane block.
At a later time it was recognized that the defoaming action of polyoxyalkylene-polysiloxane block copolymers of this type can be improved significantly by making the block copolymers water insoluble.
In German Offenlegungsschrift 2,442,853, a preparation for defoaming aqueous solutions or dispersions is described which comprises: 0.2 to 7.5% by weight of highly disperse silica or highly disperse aluminum oxide and 92.5 to 99.8% by weight of a methylpolysiloxane-polyoxyalkylene block copolymer, which contains 10 to 60% by weight of methylpolysiloxane and the polyoxyalkylene block which has more than 80% and up to 100% by weight of oxypropylene units.
The polyoxyalkylene-polysiloxane block copolymers, described in greater detail in this German Offenlegungsschrift, may also have the A'B'A' structure.
In U.S. Pat. No. 4,028,218 a method is disclosed for preventing or destroying foam in aqueous solutions or dispersions, wherein a preparation similar to that disclosed in German Offenlegungsschrift 2,443,853 is used. The preparation differs essentially in that it contains additionally an organic oil, which also has a defoaming action. Suitable organic oils are esters of alcohols and fatty acids, such as vegetable, animal or mineral oils, polybutadiene oils or polypropylene glycols.
As does U.S. Pat. No. 4,028,218, German Offenlegungsschrift 3,123,103 discloses the use of a mineral oxide-free mixture of
A) 1-20% by weight of at least one polydimethylsiloxane-polyoxyalkylene block copolymer, which comprises to the extent of 10 to 60% and preferably 15 to 40% by weight of methylsiloxane units and to the extent of 90 to 40% and preferably 85 to 60% by weight of oxyalkylene units, the oxyalkylene units comprising 75 to 100% of oxypropylene units and 0 to 25% of oxyethylene units,
B) 99-80% by weight of at least one neutral carboxylic acid ester of a saturated or unsaturated, linear or branched, monocarboxylic or dicarboxylic acid having 4 to 12 carbon atoms and a branched monohydric alcohol having 4 to 12 carbon atoms,
as defoamers in aqueous dispersions in synthetic resins.
The polyoxyalkylene-polysiloxane block copolymers, contained in an amount of 1 to 20% by weight in the mixture, may have the structure A'-[B'-A'].sub.z, with the proviso that z has a value of 1 to 3.
The silicone oils or preparations containing polyoxyalkylene-polysiloxane block copolymers, which are known from the art, are suitable, to a more or less pronounced extent, for preventing the formation of foam in aqueous solutions or aqueous dispersions of organic substance and to destroy foam already formed. It has turned out however, that aqueous dispersions of polymeric organic substances treated with such preparations have disadvantages when used, which are attributed to the addition of these defoaming agents. More particularly, it has been found that dispersions of vehicles, coating agents and adhesives, to which polysiloxanes or polyoxyalkylene-polysiloxane block copolymers have been added for the purpose of defoaming, have wetting defects when applied on surfaces. These wetting defects, which occur especially with aqueous dispersions of polyurethanes, are revealed by an irregular wetting of the substrate and lead to the formation of coatings of irregular thickness and, in the limiting case, to coatings, which have defects of different size.